our snug ride came with the insert. We used it when evidently she had already out grown it. read the labels on it if there are any. The bottom part seperated from the part that snugs in the head/arms (attached with velcro). Stopped using that part when her head control was good that it wouldnt get pulled around with each turn of the car.
Your car restraint manual should tell you specifically. But I did find this "The 2-piece support attaches together with velcro to create a single
head and body support cushion. You must use both portions for babies who
weigh between 4-5 lbs. The body support is optional but probably
helpful for most smaller newborns whose shoulders are still below the
bottom harness slots. Discontinue use of the lower portion of the insert
once the child’s shoulders are even with the bottom harness slots
*without* the insert. The upper head support insert can continue to be
used, if desired, until the harness straps are moved up to the top
harness slots."
Re: insert for snugride 35lx
HTH!